Cavitation is the formation of bubbles and cavities within a liquid stream resulting from a localized pressure drop in the liquid flow. If the pressure at some point decreases to a magnitude under which the liquid reaches the boiling point for this fluid, then a great number of vapor-filled cavities and bubbles are formed. As the pressure of the liquid then increases, vapor condensation takes place in the cavities and bubbles, and they collapse, creating very large pressure impulses and very high temperatures. According to some estimations, the temperature within the bubbles attains a magnitude on the order of 5000xc2x0 C. and a pressure of approximately 500 kg/cm2 (K. S Suslick, Science, Vol. 247, 23 March 1990, pgs. 1439-1445). Cavitation involves the entire sequence of events beginning with bubble formation through the collapse of the bubble. Because of this high energy level, cavitation has been studied for its ability to mix materials and aid in chemical reactions.
There are several different ways to produce cavitation in a fluid. The way known to most people is the cavitation resulting from a propeller blade moving at a critical speed through water. If a sufficient pressure drop occurs at the blade surface, cavitation will result. Likewise, the movement of a fluid through a restriction such as an orifice plate can also generate cavitation if the pressure drop across the orifice is sufficient. Both of these methods are commonly referred to as hydrodynamic cavitation. Cavitation may also be generated in a fluid by the use of ultrasound. A sound wave consists of compression and decompression cycles. If the pressure during the decompression cycle is low enough, bubbles may be formed. These bubbles will grow during the decompression cycle and contract or even implode during the compression cycle. The use of ultrasound to generate cavitation to enhance chemical reactions is known as Sonochemistry.
Both of these methods of cavitation to enhance mixing or aid in chemical reactions have had mixed results, mainly due to the inability to adequately control cavitation. U.S. Pat. Nos. 5,810,052, 5,931,771 and 5,937,906 to Kozyuk disclose an improved device capable of controlling the many variables associated with cavitation and the use of such a device in Sonochemical type reactions.
Metal-based materials have many industrial uses. Of relevance to the present invention are those solid state metal-based materials such as catalysts, piezoelectric materials, superconductors, electrolytes, ceramic-based products, and oxides for uses such as recording media. While these materials have been produced through normal co-precipitation means, U.S. Pat. Nos. 5,466,646 and 5,417,956 to Moser disclose the use of high shear followed by cavitation to produce metal based materials of high purity and improved nanosize. While the results disclosed in these patents are improved over the past methods of preparation, the inability to control the cavitation effects limit the results obtained.
Accordingly, the present invention is directed to a process for producing metal based solid state materials of nanostructured size and in high phase purities utilizing controlled cavitation to both create high shear and to take advantage of the energy released during bubble collapse.
The process generally comprises the steps of:
(a) mixing a metal containing solution with a precipitating agent to form a mixed solution that precipitates a product;
(b) passing said mixed solution at elevated pressure and at a velocity into a cavitation chamber, wherein said cavitation chamber has means for creating a cavitation zone and means for controlling said zone, and wherein cavitation of the mixed solution take place, forming a cavitated precipitated product;
(c) removing said cavitated precipitated product and said mixed solution from said cavitation chamber; and
(d) Separating said cavitated precipitated product from said mixed solution.
The process according to the present invention preferably employs a special apparatus to carry out step (b) in the process. Such a suitable apparatus may be found in U.S. Pat. No. 5,937,906 to Kozyuk, which disclosure is incorporated by reference herein.
The present invention is particularly suitable for producing nanophase solid state materials such as metal oxides and metals supported on metal oxides. The synthesis of nanostructured materials in high phase purities is important for obtaining pure metal oxides and metals supported on metal oxides for applications in catalytic processing and electronic and structural ceramics. The synthesis of such materials by hydrodynamic cavitation results in both nanostructured materials as well as high phase purity materials due to the fact that such processing results in high shear and high temperature local heating, applied to the synthesis stream components. High shear causes the multi-metallics to be well mixed leading to the high phase purities and nanostructured particles, and the high in situ temperatures results in decomposition of metal salts to the finished metal oxides or metals supported on metal oxides. The present invention may decompose at least some of the metal salts, and preferably all of the metal salts.
The advantage of the latter aspect is that materials produced by controlled cavitation do not require post synthesis thermal calcination to obtain the finished metal oxides while conventional methods of synthesis requires a high temperature calcination step to decompose the intermediate metal salts such as carbonates, hydroxides, chlorides etc. Such steps are often exothermic and hazardous to accomplish on an industrial scale.
The ability to synthesize advanced materials by hydrodynamic cavitation requires that the equipment used to generate cavitation have the capability to vary the type of cavitation that is instantaneously being applied to the synthesis process stream. The subject invention utilizes controlled cavitation to efficiently alter the cavitational conditions to meet the specifications of the desired material to be synthesized. The importance of the method is a capability to vary the bubble size and length of the cavitational zone, which results in a bubble collapse necessary to produce nanostructured pure phase materials. The correct type of bubble collapse provides a local shock wave and energy release to the local environment by the walls of the collapsing bubbles which provides the shear and local heating required for synthesizing pure nanostructured materials. The cavitation method enables the precise adjustment of the type of cavitation for synthesizing both pure metal oxide materials as well as metals supported on metal oxides, and slurries of pure reduced metals and metal alloys. A further capability of the method, which is important to the synthesis of materials for both catalysts and advanced materials for electronics and ceramics, is the ability to systematically vary the grain sizes by a simple alteration of the process conditions leading to cavitation. The importance of this aspect of the technology is the well known phenomena that many catalytic processes show reaction rates which are greatly accelerated by catalysts having grain sizes in the 1-10 nm range. Furthermore, materials used in ceramic as well as structural ceramics applications have been observed to densify at higher rates and to higher densities when the starting materials can be synthesized in the optimum fine grain size.
The importance of the said described invention is that it is a general method of synthesis of nanostructured materials in high phase purities while all known conventional methods of synthesis results in lower quality materials. The said invention has the capability to synthesize single metal oxides in varying grain sizes of 1-20 nm, multimetallic metal oxides in varying grain sizes and as single phase materials without the presence of any of the individual metal oxide components of the desired pure materials situated on the surface of the desired pure material. Furthermore, the synthesis of reduced metals supported on metal oxides in both grain sizes of 1-20 nm and the capability to vary the grain sizes between 1-20 nm is also possible. Due to these unique capabilities, as compared to conventional methods of synthesis, the said method affords high quality catalysts, capacitors, piezoelectrics, novel titanias, electrical and oxygen conducting metal oxides, fine grains of slurries of finely divided reduced metals, and superconductors. Conventional methods of synthesis have demonstrated the capabilities to synthesize some of these materials in high purity and fine grains; however, these processes have required a substantial adjustment in the chemistry of the synthesis of such materials. The problem with the conventional approach to the synthesis of high quality solid state materials is that the theory controlling precipitation and the chemistry of synthesis is not well understood or controllable.